Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
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MAY/JUNE 2010
WINEMAKING
Figure 6: Schematic of the antioxidant behavior of GSH in the presence of oxidized phenolic compounds. Oxidation [O] of the phenolic compound generates a quinone, which the GSH nucleophile targets to form the “grape reaction product.” Restoration of the aromatic structure occurs.
Excessive racking should be avoided, as this process always introduces some oxygen, which promotes oxidation and thus GSH depletion. Additionally, the capacity of yeast lees to absorb oxygen is well established and this limits glutathione oxidation. Paradoxically, older barrels appear to preserve GSH better than new barrels, because the oxygen level found in wine in older barrels is lower than those found in newer barrels, presumably as a result of lower porosity in the used wood. These two latter points are illustrated in Figure 7.2
Glutathione can be supplemented during fermentation through the addition of a product such as BIOAROM®, which is rich in the yeast-derived redox-active compounds glutathione, cysteine, and Nacetyl cysteine. Since yeast will simply assimilate GSH prior to 2/3 sugar depletion, supplementation should only be made in the last third of fermentation (see Figure 3).
There appears to be a threshold below which GSH supplementation is far less effective at preserving wine aroma. We estimate this to be approximately 20 ppm of GSH in-bottle. This can be seen clearly in the comparison of the aromatic profiles of wines made from the same juice (2006 Sauvignon Blanc, micro-vinification in duplicate) using Product A and BIOAROM as aroma preservatives. The total antioxidant contribution of Product A is lower
Figure 7: Comparison of glutathione content of wines handled in new and 1 year-old barrels, with and without racking.
(Table 1), and this is demonstrated by aroma preservation that is no better than the control wine (Figure 8).
It is important to understand that GSH supplementation does not enhance the yeast’s ability to generate aromas, but rather protects what aromas the yeast does produce. This can be seen clearly in Figure 9, where a control wine (Sauvignon Blanc) is compared with one made using GSH supplementation (BIOAROM®) and another made using DYNASTART®, a rehydration nutrient that is known to increase yeast aroma production.1,11,12
The most varietal wine aroma profile was produced when DYNASTART ® was used, presumably due to stimulation of yeast metabolism and enzymatic activity, and the
varietal aroma intensity of this wine exceeded the control by some margin. The wine treated with GSH supplementation during fermentation shows better preservation of aromas over the control, particularly for 3MHA (passion fruit), which is very susceptible to oxidation.
Examination of GSH concentration in the wines shows that only the wine treated with BIOAROM® had GSH above the nominal 20 ppm threshold, meaning that this wine is likely to retain these aromas during ageing to a far greater extent than the control or DYNASTART®-treated wine.
Summary
Although glutathione has been known for many years, it is only recently that its importance with respect to wine aroma preservation during ageing has become recognized. Supplementation during fermentation is shown to be an effective method of increasing wine glutathione content to ensure that a sufficient amount is available in the final wine to provide adequate aroma protection.
While glutathione supplementation is important for all white and rosé wine, it is particularly relevant for wines that rely on strong aromas for consumer appeal, and also for those being exported where time is spent in transportation and storage under conditions that may not be ideal for aroma preservation.
   
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